There is no 7.52 Mauser ammo. The left side of the 6 and the bottom of the 7 were struck lightly. The light strike on the left of the 6 makes it look kinda like a 5, but look at the top of the 6 and you can see that it's curved. You can see the marks more clearly in the pics that you posted than you can see with your naked eye, but get a magnifying glass and look at the caliber again. The same thing has happened to me before when looking at small proofs or markings on firearms. Or, if your camera has a macro feature, take a picture and blow up the image on your computer.

SAMCO Global Arms was the main seller of the M1916.

https://www.samcoglobal.com/1-1916.html

Tom